This invention relates to a paper width detecting device in a manual paper feeding tray on which a paper sheet to be fed to the mainframe of an image forming apparatus is placed.
In an image forming apparatus such as a copying machine and a printer, it is common practice to form an image on a paper sheet of various sizes. For example, in a copying machine, it is common a practice to make a copy of an original image on a transfer paper sheet of a size (A3, A4, B4, B5, etc) selected in accordance with the size of the original (A3, A4, B4, B5, etc) and the copy magnification. In this case, if the copying machine has a structure such that paper feeding cassettes of the number of kinds of paper sizes can be mounted to the apparatus mainframe, copying can be made by selecting the transfer paper of the desired size from the group of the paper feeding cassettes.
On the other hand, in a copying machine having only a limited small number of paper feeding cassettes mounted at a time, all of the papers of the desired sizes can not be selected from the group of the paper feeding cassettes. Therefore, some copying machines have a structure such that the transfer papers of the remaining sizes, which are not received in the group of the paper feeding cassettes, can be fed manually to the apparatus mainframe. In these copying machines, a manual paper feeding tray is mounted to the apparatus mainframe and the transfer papers of the remaining plural number of copy sizes are to be fed to the apparatus mainframe from this manual paper feeding tray. In this case, it is necessary for the control portion of the apparatus mainframe to be informed from the portion of the manual paper feeding tray as to which size transfer paper is set on the manual paper feeding tray so that the control portion of the apparatus mainframe may not select a wrong-sized paper.
For this reason, a mechanism for detecting the size of the placed paper is built in the manual paper feeding tray. Incidentally, paper size detection is made by detecting that the paper is of A-size, for example, from the detected value (measured value) of the width of the paper sheet placed on the tray, and further, by detecting whether it is an A3 paper placed longitudinally or an A4 paper placed transversely from the presence or absence of the placed paper at a predetermined position in the rear end side. In this case, the detection whether the placed paper sheet is present or not at the predetermined position in the rear end side is made easily by disposing a reflecting type sensor or the like at the predetermined position, however, it is not easy to detect the paper width precisely.
The conventional paper-width detecting device in the manual paper feeding tray also employs a first paper regulating member (a guide) and a second paper regulating member which are in contact with the placed paper sheet sidewise in the paper width direction in such a manner as to hold the paper sheet in between. An example of such a conventional paper-width detecting device employs a variable resistor of linear sliding type. That is, such a conventional device is provided with a linear resistor which has a fixed contact at one end and extends in the paper width direction, and is also provided with a moving contact which is in sliding contact with the linear resistor and is fixed on the first or the second paper regulating member for measuring the variation of the resistance between the fixed contact and the moving contact, so that it can detect the paper width by detecting the position of the paper regulating member. However, this structure has a problem in that the area of the resistor is made large.
This problem can be solved by employing a rotary variable resistor. For example, the paper width detector may be provided with a pair of pulleys disposed at the positions apart from each other in the direction of the paper width to which a wire is entrained, with a part of the wire wound around the shaft of the rotary variable resistor, and detects the paper width by detecting the position of the paper regulating member from the rotation of the rotary variable resistor in accordance with the movement of the paper regulating member to which the wire is fixed.
Although the above-mentioned structure employing a wire has put the small sized resistor into use, it has a problem in that the accuracy of detecting the position of the paper regulating member is made low due to the slippage between the wire and the shaft of the rotary variable resistor, resulting in the lowered accuracy of the paper width detection.
Further, mounting the rotary variable resistor to the manual paper feeding tray is conventionally achieved by soldering the terminals (leads) of the rotary variable resistor to the printed board and then fixing this printed board at the predetermined position on the base plate of the manual paper feeding tray. However, due to the dispersion of the soldering position of the rotary variable resistor to the printed board, there has been a problem in that the terminals of the rotary variable resistor are deformed or damaged by an excessive load applied from the wire when the printed board is fixed.